1. Field of the Invention
The present invention relates to flame-retardant foamed particles of a polyolefin resin, and particularly to flame-retardant foamed particles of a polyolefin resin, which are excellent in physical properties such as fusion-bonding property among the foamed particles and secondary expandability upon their molding.
2. Description of the Background Art
Polyolefin resin foams are widely used in packing materials, automobile parts, building materials and the like. The polyolefin rein foams are produced by methods such as foam extrusion, and foamed-in-place molding in which foamed particles are heated and molded in a mold. Of these, the foamed-in-place molding is used in various fields because it has such advantages that products complicated in shape can be provided.
In recent years, there has been a demand for having automobile parts and building materials have flame retardancy or self-extinguishing property. However, polyolefin resins are naturally high in combustion quality and hence easily burned by electrostatic sparks, to say nothing of a fire. For this reason, although the polyolefin resin foams are excellent in performance characteristics such as cushioning property and thermal insulating property and hence suitable for use in automobile parts, building materials, etc., their high combustion quality has become a large problem upon their use in these fields.
As a method of obtaining a flame-retardant foam, there has heretofore been known a method in which a halogen compound, phosphoric compound, metal hydroxide or the like is kneaded into a resin, and the kneaded mixture is subjected to foam pressing or foam extrusion using a decomposable blowing agent. However, this foaming process has disadvantages that a foam having a high expansion ratio is difficult to provide and that a foam complicated in shape is hard to provide as compared to the foamed-in-place molding.
As flame-retardant or self-extinguishing foams obtained by the foamed-in-place molding, there has been known a foam obtained by subjecting foamed particles obtained from a polystyrene resin with a flame retardant kneaded therein to expansion molding in a mold (the foam obtained by the foamed-in-place molding being hereinafter referred to as the "expansion-molded article"). However, the expansion-molded article of the polystyrene resin has involved a disadvantage that it is poorer in weather resistance, chemical resistance and cushioning property compared with expansion-molded articles of a polyolefin resin.
On the other hand, it has been attempted to impart flame retardancy to an expansion-molded article of a polyolefin resin by kneading a flame retardant into a raw resin for producing foamed particles in advance. However, when the flame retardant is contained in the raw resin for producing the foamed particles, the resulting foamed particles undergo shrinkage, and cells in the foamed particles tend to become fine. Therefore, such foamed particles arise such problems that they are poor in secondary expandability upon their foamed-in-place molding, and an expansion-molded article obtained from such foamed particles becomes poor in surface smoothness. The foamed particles also arise a problem that the resulting expansion-molded article becomes poor in physical properties such as mechanical strength because the fusion bonding property among the foamed particles is deteriorated.
In order to solve such problems, there has also been known a method in which a flame-retardant coating is applied to the surfaces of foamed particles of a polyolefin resin, which were obtained from a resin containing no flame retardant. However, such a method requires a process of applying a flame-retardant coating or the like to the surfaces of the foamed particles before the foamed particles are expansion-molded in a mold after the production of the foamed particles, and hence has a disadvantage that the production process of the expansion-molded article is complicated. In addition, there has been a problem that satisfactory flame retardancy is not given to the expansion-molded article because foamed particles of the polyolefin resin are generally low in acceptance of coating and adhesive property, and so the flame-retardant coating is difficult to sufficiently apply to the surfaces of the foamed particles. Further, the application of the flame-retardant coating or the like to the surfaces of the foamed particles has also offered a problem that the fusion bonding property among the foamed particles upon their molding is lowered, and so there is a tendency to provide an expansion-molded article defective in fusion bonding among the foamed particles.